1. Field of the Invention
This invention relates to a microwave phase shifter and more particularly to a hybrid coupled diode phase shifter having substantially equal values of insertion loss as the diode is switched from conducting to nonconducting states.
2. Description of the Prior Art
A microwave phase shifter is a device that is capable of changing its electrical length (insertion loss) in a predictable manner in response to a proper command signal. One of the primary applications for a microwave phase shifter is an phased array radar systems. In such phased array radar systems, the electrical length of transmission lines interconnecting parts of the system is critical. For purposes of regulating the phase of the transmitted signals in these radar systems the electrical length of the transmission lines from transmitting equipment to the several radiating elements in the antenna array must be substantially equal. A typical radar system requires, for example, several antenna systems and thousands of phase shifters. By controlling the electricl length of these phase shifters, the radar beam can be made to point in any desired direction, and this direction can be changed several thousand times per second.
Microwave phase shifters can be fabricated using either diodes or ferrites as the switched material and either can be used in coaxial, stripline, microstrip or waveguide construction. Several types of diode phase shifters have been devised such as switched line, hybrid coupled, loaded line and three element ".pi." or "T" circuits. In particular, the hybrid coupled circuit includes a 3-decibel (db)-quadrature hybrid with a pair of balanced diode switches connected to identical split arms of the hybrid. The hybrid coupled bit is used extensively because it achieves larger phase shifts while using only two diodes. The following references describe diode phase shifters and are indicative of the present state of the art: U.S. Pat. No. 3,571,762 issued Mar. 23, 1971; U.S. Pat. No. 3,400,405 issued Sept. 3, 1968; U.S. Pat. No. 3,454,906 issued July 8, 1969; U.S. Pat. No. 3,982,214 issued Sept. 21, 1976; J. F. White, "Diode Phase Shifters for Array Antennas," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-22, No. 6, June 1974, pages 658-674; R. V. Garver, "Broad-Band Diode Phase Shifters," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-20, No. 5, May 1972, Pages 314-323.
One of the undesirable characteristics of the hybrid coupled diode phase shifter is the unbalance in the insertion loss as the diodes are switched between the phase states. This unbalance results from the difference in loss produced by the diodes in the conducting state ("on") and the non-conducting state ("off"). The insertion loss is a measure of the change in power (amplitude) between the RF input and output of the phase shifter. It is desirable that the amplitude of the output be the same as that of the input and thus the difference in insertion loss between the diode switching states be zero. A phase shifter having substantially equal insertion loss between the phase shifter states is desirable for wide frequency band operation and small phase and amplitude errors for more accurately steering the antenna beam in the radar system.